fix(ci): drop netcoreapp3.1 from dotnet workflow (#87)

.github/workflows/dotnet.yml

@@ -23,7 +23,6 @@ jobs:
       uses: actions/setup-dotnet@v3
       with:
         dotnet-version: |
-          3.1.x
           6.0.x
           7.0.x
 
@@ -36,9 +35,6 @@ jobs:
     - name: Build
       run: dotnet build --no-restore --configuration Release
 
-    - name: Test .NET Core 3.1
-      run: dotnet test --no-build --verbosity normal --configuration Release --framework netcoreapp3.1
-
     - name: Test .NET 6
       run: dotnet test --no-build --verbosity normal --configuration Release --framework net6.0